1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More specifically, the invention relates to a fail detection system for detecting failure of an electromagnetic actuator employed in the active suspension system and fail-safe operation to be performed in response to failure of the actuator.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987, which has been assigned to the common owner to the present invention, discloses a typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit which controls the valve position. The valve position of the pressure control valve is controlled by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and whereby controlling suspension characteristics.
On the other hand, European Patents 0 283 004, 0 285 153 and 0 284 053 disclose technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
In the above-mentioned prior proposals, an electromagnetic actuator, e.g. solenoid, is employed in cooperation with the pressure control valve for controlling the valve position of the pressure control valve for adjusting delivery of working fluid to the hydraulic chamber in order to perform attitude change suppressive control, such as anti-rolling, anti-pitching, bouncing control and so forth, and to absorb vibration to be transmitted to the vehicular body. In such an active suspension system, substantially high response of the actuator is required in order to provide satisfactorily high response in adjustment of suspension characteristics depending upon the vehicle driving condition. For providing sufficiently high response to the electromagnetic actuator, there has been proposed to superimpose dither signal to a suspension control command for the actuator. Such a dither signal is effective to provide sufficiently high response to the actuator and thus successful in enhancement of response characteristics of the active suspension system.
However, superimposition of the dither signal may cause difficulty in detecting failure of the actuator. Namely, failure of the actuator may be detected by comparing a suspension control command value issued by a control unit with a driver current actually supplied to the actuator. In practice, the actually applied driver current cannot correspond to the suspension control command value due to inductance of the actuator. Furthermore, tolerance in elements forming the current feedback circuit may influence the driver current. Particularly, when the dither signal is superimposed on the suspension control command, tolerance or fluctuation of high frequency response characteristics of respective circuit elements becomes substantial. Therefore, in fail detection, a failure detection criterion to be compared with a difference or error between the suspension control command value and the actually applied driver current cannot be set at a satisfactorily small value. This leads to a lowering of the precision level in the detection of a failure.
In case of the active suspension system, even small error between the suspension control command value and the actually applied driver current may cause substantial degradation of the suspension control characteristics.